Our laboratorys interest has been in studying the interaction between CD8+ T cells (known also as cytotoxic T lymphocytes, or CTL) and their cellular targets. In particular, we have focused on how the T cell receptor (TcR) of CTL interacts with class I molecules of the Major Histocompatibility Complex (MHC), with special attention given to cross-reactivity- the ability of a TcR to recognize dissimilar antigenic complexes. Antigenic complexes are formed when MHC molecules bind short protein fragments, derived from pathogen proteins, and display them on the cell surface. Molecular mimicry has often been invoked explain cross-reactivity-- this model suggests that the contacted areas of different complexes recognized by the same TcR are likely to be similar in shape, charge, or both, in spite of the primary sequence differences. To examine the mechanism of cross-recognition in a model system we developed, crystal structures were completed for two different ligands recognized by the same TcR, through collaboration with Drs. E. Collins and R. Zhao at the University of N. Carolina. The crystal structures show that the molecular surfaces of the two class I complexes are significantly different in shape and charge distribution. These structures and our functional data together suggest that rather than through molecular mimicry, this TcR recognizes these two different ligands using an array of common and differential contacts. In other studies, we continued to examine the CTL response to the peptide antigen MART1, presented by class I molecules on the surface of melanoma cells. Recently, we identified a superagonist analogue of the MART-1 peptide containing a Leu in position 1 (LAGIGILTV; 1L) which elicited patient CTL in vitro having superior efficacy compared to CTL raised using the native MART1 peptide. These data provide a rationale for exploring the use of superagonist analogues of tumor antigens for inducing CTL capable of mediate tumor destruction in vivo. Presently, we are continuing work on the optimization of peptide epitopes for targeted immunotherapy, emphasizing the development of superagonist ligands in other systems other than MART1. We also are carrying out proteomic studies of differential T cell signaling triggered by engagement of MHC class I/peptide complexes having partial agonist, antagonist, and superagonist activity. - TcR interactions, CTL analysis, HLA-A2, MART-1 tumor antigen, peptide/MHC complexes,